This invention relates to a temperature-measuring apparatus for determining temperature with the aid of a thermo-sensitive element.
Hitherto, electronic measurement of, for example, body temperature has been undertaken by a clinical thermometer having such an arrangement as is shown in FIG. 1. The clinical thermometer is provided with a bridge circuit 1 formed of a thermistor Rx acting as a thermo-sensitive resistor element, and resistors R1, R2 and R3. Resistors R4 and R5 are used as d.c. bias resistors. The resistances of all these resistors are so preset as the cause voltage Eg across the output terminals A and B of the bridge circuit 1 to indicate 0 at a initial temperature T.sub.T , for example, 32.degree. C. When the resistance of the thermistor Rx varies with body temperature, then an unbalanced voltage Eg appearing between the output terminals A and B of the bridge circuit 1 is impressed on an analog-digital converter 2, which converts said voltage into a digital signal, for example, a binary decimal code. An output signal from said A/D converter 2 is conducted to a display device 4 through a decoder 3, to indicate the measured body temperature in the form of digits.
Where, with the prior art temperature measuring apparatus, a referential voltage impressed between the power supply terminals of the bridge circuit 1 is denoted by V.sub.R, then an output voltage Eg from said bridge circuit 1 may be expressed by the following equation (assuming R4 = R5 = 0): ##EQU1##
Since the body temperature T and the resistance of the thermistor Rx are in inverse proportion, an output voltage Eg from the bridge circuit 1 have a relationship shown in a solid line A in FIG. 2. Where the referential voltage V.sub.R changes, for example, increases, then the output voltage Eg and body temperature present a relationship indicated in a broken line B in FIG. 2 which passes through a point representing the initial body temperature T.sub.I at a different gradient from the solid line A with Eg taken to be 0.
Where not only the referential voltage V.sub.R but also the resistances of the resistors R1, R2 and R3 change, then the output voltage Eg and Body temperature indicate a relationship illustrated by another broken line C which does not pass through the point of the initial temperature T.sub.I but shows a different gradient from the above-mentioned lines A, B. Thus, the body temperature can not be correctly measured by variations in the referential voltage V.sub.R and the resistances of the resistors R1, R2, R3.
The A/D converter 2 is generally of a dual slope type. The offset voltage of an operation amlifier used with said dual slope type A/D converter causes variation in the initial temperature T.sub.I, leading to errors of measurement.